Machine tool with outboard support for drill

ABSTRACT

An automatic drilling machine is afforded in which the drill is automatically retracted when excessive resistance is encountered, and upon retraction may be automatically cleaned, lubricated and cooled. At no time is more than one-half the length of the drill unsupported.

United States Patent [1 1 Filed: July 14, 1970 Appl. No; 54,870

Related US. Application Data Division of Ser. No. 752,200, Aug. 13, 1968, Pat. No. 3,584,524.

US. Cl 408/234, 408/17, 408/97, 408/56, 408/705 Int. Cl... 1323b 41/02, B23b 47/32, B23b 49/00 Field of Search 408/17, 97, 234, 408/705, 110, 57

References Cited UNITEDSTATES PATENTS 5/1950 Cudini 408/97 X Langebach Nov. 27, 1973 [54] MACHINE TOOL WITH OUTBOARD 2,536,333 l/195l Waxelbaum 408/705 2,552,463 5/1951 Searles 408/56 SUPPORT FOR DRILL 2,922,323 H1960 Weidner 408/17 [75 Inventor: Bob J. Langebach, Athens, Ohio Assignee: Abex Corporation, New York, Primary Examiner-Francis H9531 ABSTRACT An automatic drilling machine is afforded in which the drill is automatically retracted when excessive resistance is encountered, and upon retraction may be automatically cleaned, lubricated and cooled. At no time is more than one-half the length of the drill unsupported.

2 Claims, 30 Drawing Figures PATENTEDNUVZ? I375 3,775,021

SHEET OlUF 15 RATENIEUHUY 27 1975 3. 775,021

SHEET DSUF 15 PATENTED NOV 2 7 i975 SHEET USUF 15 PATENTEUNUVZY I975 3.775.021 SHEET 070F 15 PATENTEUNUVZ? I975 SHEET 080F 15 FIG. IO 2/2 PATENTEDHBVZT 1975 3,775,021

SHEET 110F 15 MACHINE TOOL WITH OUTBOARD SUPPORT FOR DRILL This application is a division of application Ser. No. 752,200, filed Aug. 13, 1968 and now U.S. Pat. No. 3,584,524.

This invention relates to a machine tool which presents a drill to a work piece, and in particular to a machine tool for drilling vent holes in tire molds.

Automotive tires, as well as tires for heavy duty earth-moving equipment, are vulcanized in tire molds which present the pattern for the tire tread, the various diameters of the tire and the sipe plates which are responsible for forming thin slits in the tread of the tire for improving performance of the tire. The actual mold construction requires vent holes for venting the mold in the course of vulcanizing the rubber. Superior tire molds are cast or forged from metal, and the present practice is to drill the vent holes by manually operated power drills. The drills, or bits as they are sometimes called, are expensive and a great deal of breakage occurs which not only increases the cost of production, but which also slows the rate of production as an additional cost factor.

The primary object of the present invention is to lower the cost of producing tire molds and to do this by a machine tool which greatly diminishes the likelihood of broken drills, which enables a plurality of tire molds to be drilled simultaneously and fully automatically, and to support the drill by a head index which affords universal movement of the drill motor so that the drill can work at any diameter and any angle, and can be readily switched from a position where it works on one wall of the mold to a position where it will work on an opposed wall of the mold.

Further objects of the present invention are to construct a machine tool of the automatic drill type which can also be used for performing operations similar to the foregoing on work pieces other than tire molds, to automatically lubricate and clean the tip of the drill, to retract the drill in the event the work piece presents an unacceptable high resistance likely to break the drill, and to efficiently move the drill in opposed reciprocal directions incidental to performing work. Yet further objects of the present invention are to incorporate in the machine tool sensing means and memory means which will detect critical conditions and circumstances for controlling the machine, to lend constant support to the intermediate length of the drill while it is working, and to so construct the machine tool as to achieve highly efficient operation.

Other and further objects of the present invention will be apparent from the following description and claims and are illustrated in the accompanying drawings which, byway of illustration, show a preferred embodiment of the present invention and the principal thereof and what I now consider to be the best mode in which I have contemplated applying that principle. Other embodimens of the invention embodying the same or equivalent principle may be used and structural changes may be made as desired by those skilled in the art without departing from the present invention.

In the drawings:

FIG. 1 is a perspective view of a typical tire mold;

FIG. 1A is an enlarged fragmentary view of a portion of the mold shown in FIG. 1;

FIG. 1B is a perspective view of the head index;

FIG. 2 is a plan view of the machine tool;

FIG. 3 is a sectional view substantially on the line 3-3 of FIG. 2;

FIG. 4 is a side elevation on the line 44 of FIG. 3;

FIG. 5 is a sectional view on an enlarged scale of a fragmentary portion of the table on which the mold will be supported;

FIG. 6 is an elevation partly in section of the motor support and related parts;

FIG. 7 is another section of the motor support means and related parts;

FIG. 7A is a fragmentary elevation showing the retracted position of the drill; 4

FIG. 8 is an end elevation substantially on the line 88 of FIG. 6;

FIG. 9 is an end view on the line 99 of FIG. 10;

FIG. 10 is a fragmentary elevation partly in section on the line 10-10 of FIG. 9;

FIG. 11 is a plan view partly in section of a portion of the support means for the drill motor;

FIG. 12 is a fragmentary elevation, partly in section, of the head index and drill head;

FIG. 13 is a framentary sectional view of the drill head;

FIG. 14 is a top plan view of the head index;

FIG. 15 is a plan view of certain parts used for adjustment;

FIG. 16 is a fragmentary sectional view of the head index;

FIG. 17 is an end elevation substantially on the line ll717 of FIG. 16;

FIG. 18 is a fragmentary sectional view of a column support;

FIGS. 19, 20, 21 and 22 are sectional views taken substantially and respectively on the lines 19-49, 20-20, 2121 and 2222 of FIG. 18, but on an enlarged scale;

FIG. 23 is an elevation of certain hydraulic control Parts;

FIG. 24 is a diagrammatic plan of hydraulic circuitry and related parts; and

4 FIGS. 25, 26 and 27 are views of portions of the hydraulic circuit using JIC symbols.

GENERAL DESCRIPTION While the machine tool of the present invention 35, FIG. 2, is specifically constructed to drill vent holes in a tire mold as TM, FIG. 1, it will be appreciated that other work pieces may be drilled by the present machine tool. In any event, the machine preferably includes two rotary tables 36, FIG. 2, on which two tire molds may be set and clamped in place, each table 36 including a ring gear 37, FIG. 5, adapted to be turned or indexed through increments by a pinion 38 driven by a table motor M.

There are also two drill assemblies each presenting a drill bit 40, FIG. 2, and a drill motor 42, FIG. 3. The drill motor is supported beneath a head index 44, and the head index in turn is carried by an over-arm 45 which can be raised and lowered and which can also be turned both to position the drill and to displace the drill from position above the table to enable the tire molds to be moved into position.

The head index 44 includes a handle I-Ill, FIG. 4, which when loosened, enables the head index to be turned about a horizontal axis through a handle H2 to vary the angle of the drill insofar as working on a particular radius may be concerned.

The head index also includes a knob or handle l-I3, FIG. 16, which controls an eccentric, the purpose of which will be explained hereinafter.

Referring to FIGS. 6 and 7A, the drill motor 42 is supported for advancement and retraction on a common reciprocal axis. When the drill is fully retracted, FIG. 7A, means are effective to furnish lubricant to the tip of the drill and to furnish air to the drill for cleaning chips or tumings therefrom. A tachometer 48, FIG. 24, senses the rpm of the drill, and this tachometer in conjunction with other means to be described may at times sense conditions which manifest unacceptable high resistance of the work piece to the drill, whereupon the drill is retracted incidental to a new start and at the same time the tip of the drill is lubricated and cleaned.

Because the vent holes in a tire mold are relatively small, a necessarily small drill from the standpoint of diameter is used, but at the same time, a long drill bit is necessary. Therefore, the drill bit is constantly supported at its intermediate length in a positioning assembly 49, FIG. 7A, and this same positioning assembly is so constructed as to present the passages for feeding lubricant and air to the drill, which is an automatic occurrence at the time of full retraction of the drill whether retraction occurs as an incident to work piece resistance referred to above or whether retraction be at the end of a completed work cycle in which the vent hole is completed.

Additional sensing means in the form of a linear potentiometer 50, FIG. 24, is used to determine when the drill has made a complete penetration so far as concerns completion of a drilled opening and also to sense when the drill attains a fully retracted position. Thus the linear potentiometer is a sensing means for controlling operations to be described hereinafter, and it should also be observed that the machine is equippped with a memory unit in the form of a cam 51, FIG. 11, which serves to constantly register the forwardmost position of the drill. Thus the cam 51 moves with the drill and its position is a measure of the amount of penetration of the drill into the work piece. If the drill is to be retracted because of exceptionally high resistance in the work piece, retraction as well as the subsequent stroke for a new start is to occur at a high rate of feed. This increases the production rate of the machine tool, but nevertheless, at the time of actual drilling feed is to occur at a more slow rate. The memory device 51 is effective to control these feed rates, and when the drill attains its former working position on a return stroke following intermediate retraction, the cam or memory device 51 is effective to reestablish a slow rate of feed or advancing rate of the drill.

The tire mold TM, FIG. 1, was initially present as a casting presenting the general contours. Then it was shaped on a boring mill in which were provided the various stepped diameters as D1 and D2 apparent in FIG. 1. Thereafter the mold was further completed by engraving techniques incidental to forming the various tread details TR, and following this the so-called sipes SP, FIG. 1A, were formed.

The present invention is concerned with one of the final stages of completion of the mold in which vent openings V0, FIG. 1A, are formed in the walls W and W1 of the mold which are necessary to proper vulcanization in the course of using the mold for curing the rubber which will form the tire. It should be noted that walls W and W] are really opposed walls in the sense that wall W faces inward whereas wall W1 faces outward. To drill the front wall W, the drill will point outward as shown in FIG. 2, but to drill the back wall W1, the drill will have to be turned 180 to point inward toward wall W1.

Heretofore these vent openings were completed by hand operation which is a slow and tedious process, and moreover, a great deal of cost was encountered as the result of breaking the drills. Thus, in hand operation the workman himself manipulates a portable hand tool which rotates the drill bit, but when resistance is encountered in the course of forming a vent opening, the workman cannot necessarily sense the circumstance where resistance to torque is so severe as to produce a broken drill bit.

Furthermore, in the instance of manual drilling, no practical means is available to lend support to the long portion of the drill bit between the end portion gripped in the chuck and the drill end which is performing the desired drilling operation. Under the present invention, a constant support is presented so that at no time is more than one-half the length of the drill bit unsupported.

While the cost of replacing drill bits may itself seem minimal compared to the cost of the present machine, a broken drill bit under manual operation is only a manifestation of slow production. Thus, not only is time consumed in extracting the broken bit and replacing a new one, but necessarily the workman will operate the manual tool at a slower rate in an effort to avoid a broken bit which itself can be dangerous. Thus, the time element is one factor of several contributing to high cost all of which are obviated under the present invention which entails a machine which will enable tire molds to be produced at a rate about three times that heretofore so far as concerns the drilling of vent holes in the tire mold.

Referring to FIG. 2, the bed of the machine is capable of handling two work pieces simultaneously, one being supported on each table 36. The two tables are identical in construction and operation, and this is equally true of the drills, their operation and their supporting means. Therefore, the construction of the machine will be described in terms applicable to either table 36 and either drill unit.

THE SUPPORTING COLUMN As shown in FIG. 3, which illustrates in end elevation one-half the whole construction of the machine as viewed in FIG. 2, it will be observed that the drill motor 42 depends from a drill head 55 normally positioned above the table 36 and itself depends from the head index 44 which in turn is connected to arm 45 supported in cantilever fashion at the upper end of a sleeve-like column support 70. As will be described hereinafter, the drill head may be turned 180 horizontally about a vertical axis to accommodate the requirement that either of two opposed walls of the mold may have to be drilled with vent holes. Furthermore, since the drill may be required to work at several different diameters of one wall, the head index itself may be tilted about a horizontal axis so that the drill may engage a wall of the mold from virtually any selected angle.

The horizonal arm 45, FIG. 18, is clamped in a collar 72, which in turn is pinned to the upper end of the vertical support column so that the arm 45 can be both swung and elevated or lowered with corresponding movements of the sleeve 70. The sleeve 70 includes a key 70K adapted to fit into a key slot 758 formed in a collar 75 which itself is clamped to the main stationary horizontal bed 76, FIG. 4, of the machine tool.

The column 70, FIG. 18, extends through and beyond the stationary collar 75 and is provided at the lower end with a fixed nut 80. The screw 81 is extended through the nut, coaxial with column 70. The lower end of the screw 81 is supported for rotation in a thrust bearing block 82. The screw adjacent the lower end is provided with a sprocket 83 adapted to be turned by a chain 85, the chain 85 in turn being driven by a sprocket 86 representing the output or drive of a reversible, two speed (variable speed) motor M1, FIG. 22. Thus, when the motor M1 is energized the screw 81 will turn, and depending upon the direction of rotation, the sleeve 70 will be accordingly raised or lowered.

Assuming the circumstance to be that shown in FIG. 2 where the drill head 55 and head index 44 are centered over the table T and that tire molds are to be set in position, the motor M will be so driven as to turn the screw 81 to raise shaft 70 eventually disengaging the key 70K from the key slot in the collar 75. It is then possible to turn each arm 45 to a position which exposes the tables T so that the tire molds may be set thereon.

Referring to FIG. 5, the table 36 is bolted to the ring gear 37 by which the molds or other workpieces to be drilled may be indexed to the various radial positions where vent holes are to be drilled therein. The ring gear 37 is driven through the pinion 38 in turn driven by the table motor M. As also shown in FIG. 5, the bed 76 serves to support a roller bearing assembly 87 which anti-frictionally supports the table 36.

The table 36 is formed with a plurality of radially extended slots 88, FIGS. 2 and 5, and these slots serve as guides for the heads of a plurality of T-bolts (not shown) which serve to rigidly clamp the tire mold to the table 36. After a tire mold has been set in place on the table T, the mold is trued up or accurately oriented through the media of indices on the table, although other registering means may be used. This initial positioning is merely one of orientation, and has nothing to do with the true starting position of which reference will be made hereinafter.

After the molds have been clamped on the corresponding tables, the arms 45 are swung back into position over the mold and the key 70K is approximately aligned with the key slot 75S. The motor M1 is then reversed to in effect retract the nut 80 causing the sleeve 70 to be gradually lowered. It will be observed that the key slot 75S is formed to present a rather wide opening, almost 90 in arc, and the sides thereof are sloped downward and inward so that as the column 70 is lowered the key 70K will be accurately guided into the narrow portion of the key slot into which it neatly fits. Thus the key is returned to the key slot whereby sleeve 70 is in effect locked in position against rotative motion and of course the friction presented by the idled motor M1 prevents any rotation of screw 81 and consequent vertical movement of column 70.

It may be mentioned that the screw 81 will in effect be stopped when the drill is lowered to the desired operating diameter of the work piece on table 36.

Means are provided to lock column 70 in the position to which it was lowered as just described, and such means are operated by a piston 90 in an air cylinder 91, FIGS. 3, 18, 20 and 21. The cylinder 91 is supported at the underside of the bed 76 of the machine, as shown in FIG. 3, and its piston 90 is pivotally connected to the lower end of a brake operating arm 92. The operating arm 92 has a shaft 93 fixed thereto, and this shaft is turned to present a pair of left and right hand screws 94 and 95 as shown in FIG. 21. These screws in turn are mounted in internally threaded bores in a pair of brake segments 94A and 95A free to slide within the collar 75 which is clamped to the bed of the machine as mentioned above. The brake shoes or segments 94A and 95A are formed with curved faces 94B and 95B adapted to fit a portion of the outer circumferential of column 70.

When the column is to be raised to disengage key 70K from the key slot S, the piston is retracted and the two screws 94 and are turned to free the brake elements 94A and 95A from shaft 70. After the shaft or column 70 has been lowered to its working position described above, the air cylinder is actuated to advance piston 90 which, in effect, tightens the two brake elements on column 70 thereby holding column 70 in a fully locked position.

THE HEAD INDEX As noted above, the head index 44 may be turned on a horizontal axis to present the drill bit to the diameter of the wall of the mold where vent openings are to be formed in the desired spaced relationship in a circumferential sense. There are, of course, circumstances where the drill can be presented to the desired diameter merely by raising or lowering column 70. Yet further, there may be circumstances where different diameters may be worked on by the drill merely by positioning the head index about its horizontal axis. But more likely than not, the configuration of the tire mold may present a geometry which will match either circumstance alone, requiring vertical movement of the column 70 as well as tilting of the head index about its horizontal axis in order to match the arc of the tire mold presenting the diameter where the vent holes are to'be drilled.

Referring to FIG. 16, a fully locked position of the head index 44 is normally maintained by handle H1 in locked position. When the head index is to be moved, handle H1 is appropriately turned, turning a screw shaft 98 having a threaded connection to the head 99 of a sleeve-like clamp 100 which at one end is formed with a rabbeted shoulder 101 normally in tight engagement with one face 102 of a worm wheel 105 having a worm thread 106. The sleeve 100 fits within the head 44 and the two are non-rotatably related by bolts 107 having heads l07I-I mounted in the outer face 44F of the head 44. It will be observed in FIG. 16 that there is a slight amount of clearance between the underside of the bolt heads 107I-I and the bottom of the sockets in which they are located.

The worm wheel 105 is fixed and stationary at all times. Thus, the worm wheel, FIG. 16, is secured against rotation by bolts 110 to one of the end faces 45F of the over-arm 45, and when the shoulder 101 of sleeve 100 is drawn up tightly against the worm wheel by a tightening turn of handle I-Il, sleeve 100 in effect locks the head index 44 through bolts 107. That is, the head index is locked in a rotative sense, although the bolts 107 are free to move axially enabling the locking sleeve 100 to be moved by screw 98 in an axial sense as viewed in FIG. 16 incidental to clamping and unclamping the sleeve 100 with respect to the worm wheel 105. The worm wheel, as a stationary gear fixed to the arm 45, is engaged by a travelling gear carried by the head index, to adjust the head index as will now be described.

When the sleeve 100 is thus released with respect to the worm wheel, it is then possible to use handle H2 to rotate the head index 44 circumferentially about the axis of screw 98 incidental to tilting the drill 40 to a different angle. To this end, a shaft 120, FIG. 15, is arranged to rotate with the wheel of handle H2 and extends longitudinally of the head index parallel to the lock sleeve 100. The end of shaft 120 opposite the handle H2 carries a bevel gear 121 normally meshed with a bevel gear 122. The bevel gear 122 is carried on a shaft 123 which in turn has a worm gear 125 fixed thereto. Both shaft 120 and shaft 123 are supported for rotation within the head index.

The worm gear 125 is a head index adjusting gear and is meshed with the worm wheel 105. The worm wheel, being fixed, constitutes a rotary track for the worm gear. Thus, when sleeve 100 has been freed as above described, handle H2 may then be turned, turning shaft 123 through the engaged bevel gears 121 and 122, rotating the worm gear. The worm uses the worm wheel as a track and accordingly the head index 44, FIG. 17, is forced to turn about axis XX, FIGS. 16 and 17, as a center. In this way the drill 40, FIG. 4, may be tilted at an angle other than horizontal. When the desired angle for the drill bit has thus been achieved, handle H1 is then turned in a tightening direction bringing the rabbeted shoulder 101 inwardly tight against the worm wheel and thereby locking the head index against any further rotative motion.

It was mentioned above that the tire mold may present a back wall W1 to be formed with vent openings. This would entail having the drill 40 point toward the observer, rather than away from the observer, as viewed in FIG. 3. To enable this to be accomplished, means are provided to first drop slightly the supporting head 130 which carries the drill unit. Thus, and again referring to FIGS. 14 through 17, the same shaft 120 which carries the bevel gear 121 also carries a bevel gear 133 adapted to be meshed with a bevel gear 134 carried on a shaft 135 disposed parallel to shaft 123 within the head index.

Shaft 120 is adapted to slide to disengage bevel gear 121 from bevel gear 123 while establishing engagement between bevel gears 133 and 134. To this end, a sleeve 140, FIG. 15, having a medial groove 141 therein is located on shaft 120 between the two bevel gears 121 and 133. Shaft 120 carries a spacer 120C. A set screw 120A normally holds collar 120C stationary.

A stud 150, FIG. 16, is positioned eccentrically at the lower end of a positioning shaft 151 fixed to handle H3 to rotate therewith. Stud 150 reposes in groove 141 of sleeve 140. The position of the handle H3 is one of two positions, either position being normally maintained by a spring loaded ball detent 152. Thus by turning handle H3 with sufficient force to overcome the holding action of the spring detent, shaft 151 will turn and the eccentric stud 150 will shift collar 140 and shaft 120 to engage the two bevel gears 133 and 134. Then when sleeve 100 has been unlocked through appropriate turning of handle H1, turning of handle H2 will cause bevel gear 133 to rotate bevel gear 134, turning shaft 135.

Shaft carries a pinion 161, FIG. 16, for adjusting the tool support head. The pinion 161 is meshed with a ring gear 170, FIG. 14, having external teeth T, FIG. 16, engaged by the teeth of pinion 161 and an internal thread 170A related to a nut 171 carried at the upper end of the tool support head 55. The ring gear 170 is rigidly secured to the underside of the head index by a bolted clamp ring 172. Ordinarily bevel gears 133 and 134 are disengaged with the shoulder 175 of the drill head 55 drawn up tightly against the underside of the clamp ring 172. When, however, the two gears 133 and 134 are meshed and handle H2 appropriately turned, rotation of the tool support head adjusting gear 170, through nut 171, causes the head 55 to be lowered, producing separation between the shoulder 175 and the clamp ring 172. This being so, the drill support head 55 is in efiect freed from the head index 44 and may be turned through and again appropriately spaced locking or holding detents (not shown) are provided between the head index and the drill head to position the drill unit in either of one of two selected 180 positions.

The height, angularity and working direction of the drill having now been set, the table T holding the work piece can be indexed to its zero or starting position. This is accomplished through a so-called jog operation, the starting position for the first vent hole being a predetermined one depending upon specifications. The present machine is equipped with a panel P at the front of the machine housing, FIG. 2, on which is located the various buttons for sequenching and automatic operation, and the table may be positioned as an incident to depressing the so-called rapid jog button (150 inches per minute) or a jog feed button which itself can be set for any increment of table feed up to 150 inches per minute. There is also a jog increment feed button, providing 0.005 inches of are for each actuation of the jog increment button. Details of this aspect of the pres ent disclosure are not necessary to an understanding of the mechanical functions involved since the ultimate effect of jogging is to energize the table drive motor M to produce increments of rotation of the pinion 38, FIG. 5.

In any event, the table 36 is indexed to its start position, and under this circumstance the point of the drill 40 is opposite the precise point of the mold wall that is to be penetrated by the drill bit in forming the first of many vent openings.

THE DRILL HEAD Details of the drill head or unit are illustrated in FIGS. 6 through 13. In FIGS. 6, 7 and 10, the drill unit is shown under the circumstance where the drill 40 has been advanced to its forwardmost penetrating position (high limit) with regard to the tire mold. In FIG. 7A, the drill unit has been fully retracted (low limit). During reciprocal movement of the drill, the drill head 55 remains stationary, of course, and serves principally as a guide for a drill motor support carriage as will be explained. In this connection it will be observed that the drill bit is constantly supported by two support bushings 200 and 201, FIG. 10. The outer support bushing 200 is part of an end support 205 which is fastened to a pair of parallel, laterally spaced guide rods 210, FIG. 7, resort being had to screws 212 for thus es- 

1. In a machine tool characterized by a motor driven drill carried in a reciprocal carriage supported in turn by a support head, guide means on the support head and on which the carriage is slidably supported for reciprocal motion, means to reciprocate the carriage to advance and retract the drill motor, a fixed bushing support attached to and projecting outboard of the support head and presenting a first bushing having a fixed location for embracing the drill, a slide on said guide means having a support presenting a movable second bushing embracing the drill, means to shift the slide with and as an incident to reciprocation of the carriage thereby to move the second bushing so that at no time during its advancing stroke is more than onehalf the length of the drill unsupported, said means to shift the slide including a cross plate on the carriage, an attachment on the slide presenting a pair of stops in the path of the cross plate, said stops being spaced from one another and separated from one another by a distance greater than the width of the cross plate, the cross plate in its advancing movement engaging one stop to advance the slide and the second bushing and in its retracting movement engaging the other stop to retract the slide and the second bushing, the spacing between the stops determining the points where the cross plate engaged with a stop advances and retracts the second bushing, said fixed bushing being supported by said guide means.
 2. A machine tool according to claim 1 in which the tip of the drill is presented to the first bushing in the fully retracted position of the drill, said fixed bushing support and its bushing being formed wiTh first and second passages respectively enabling lubricant and air under pressure to be furnished to the tip of the retracted drill. 